This invention relates to a hermetically sealed container having a plastic container body and a metallic top and, more particularly, to a plastic container body having a peripheral flange at its open end adapted for closure and hermetic sealing by double-seaming.
Receptacles or can bodies are conventionally closed by metal top and seaming operations. Single seams may be used in conjunction with special chemical sealants to avoid leaks, but are not effective in the absence of the sealant to effectively render a container leak-proof. The application of such sealants requires clearance of the materials for toxicity and the like where the container is intended for edible substances. In double-seaming, a metal top is generally provided with a flange which is folded twice to provide two seams to capture a portion of the container thereby sealing the container.
More recently, plastic containers with reduced gas permeability are being developed for the storage of food and beverages. Such containers have a laminated or multilayered structure that is generally formed from one or more layers each of a thermoplastic material and a material having good gas-barrier properties. The thermoplastic materials are generally resilient, commonly used materials being, for example, polyethylene and polypropylene, which may be used to form both the inner and outer layers of the container. An interior gas-barrier layer is generally formed by a material such as polyamide, acrylonitrile, polyvinylidene chloride and ethylenevinyl alcohol copolymers.
When double seaming is used in an attempt to seal metal closures to such laminated multilayered thermoplastic container bodies, the containers frequently present leakage problems due to a number of reasons, but especially due to the tendency of the proximate plastic material to pull out of the metal closure. Moreover there is a marked tendency for the metal closure to simply cut into the plastic sidewall during the seaming operation. In addition, when such plastic containers are accidently dropped, they are frequently deformed to such an extent that their contents leak. In addition, the dimensions of plastic containers vary greatly from their standard shape because of their nature and the method by which they are manufactured. Attempts to close such containers by double-seaming metal ends frequently fail to achieve a reliable closure and seal because of the substantial variation in size of the flange formed in the plastic container. As a result, there is a substantial variation in the integrity and reliability of the seal obtained with plastic containers. In addition to these problems, efforts to double seam laminated container bodies have frequently resulted in fractures and cracks as well as delamination of the container body during the double-seaming operation because of the lack of flexibility and resiliency of the laminated container body to withstand the severe stresses encountered during double-seaming. U.S. Pat. No. 3,685,685 describes a plastic-metal container that is formed by double-seaming. In this double-seamed configuration, the plastic flange of the container is compressed at least to one-half the original thickness to form a seal. U.S. Pat. No. 3,923,190 shows one attempt to solve this problem in which a polymeric flange is double seamed to a metal closure. Moreover, U.S. Pat. No. 4,398,648 relates to a novel flange configuration that is useful in forming a good seam with metal closures, the configuration being effectively interfoldable with a standard metal closure to provide leak-proof containers.
A reliable seal is necessary for the container to be usable to store foods and beverages in a non-frozen or unrefrigerated state. The lack of seal integrity permits oxygen to enter the container and results in deterioration of any foodstuffs or beverages within the container which defeats the purpose of the included gas-barrier lamination.